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THE 

SLIDE VALVE, 

SIMPLY EXPLAINED. 
By W/J. TENNANT, Asso. M.I.M.E 

REVISED AND MUCH ENLARGED 

By J. H. KINEALY, D. E., 

Professor of Mechanical Engineering in Washington University ; 

M. Am. Soc. Mech. Engrs. Author of "Steam Engines and 

Boilers ; " ** Low Pressure Steam Heating Charts." 



FULLY ILLUSTRATED WITH ORIGINAL DRAWINGS AND DIAGRAMS. 



SPON & CHAMBERLAIN, 

12 CORTLANDT STREET, NEW YORK. 

1899. 




41312 



Entered according to Act of Congress in the year 1899, 

By SPON & CHAMBERLAIN, 

in the Office of the Librarian of Congress, Washington, D. C. 



fQC0Pi£b *ec&n 




^ WFBuWR PRINTING HOUSE 



INTING HOUSE, NEW YORK, U. S. A. 







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PREFACE. 



This work is based upon notes and diagrams 
which were prepared by the writer with no 
more ambitious object, originally, than to help 
his railway students toward the obtainment of 
clear general notions upon the important sub- 
ject of the slide valve. 

For practical demonstration of the functions 
and operation of the slide valve, a large sec- 
tional model engine, with provision for varia- 
tion of the operation of its parts, is very valu- 
able ; but it seems to the writer to be desirable 
that each student should have in his own pos- 
session a model to cogitate upon, and that his 
model should give him, graphically, results 
which should be not merely qualitative, but 
also, in some degree, quantitative, to enable 
him to institute comparisons between the 
actions of different valves operated ulftftr vatfy* >* 
ing conditions. With this end in view, the 
writer conceived the idea of using on a base- 



iv Preface. 

board a rotary disc to represent a crank-shaft, 
together with the idea of obtaining concentric 
circular diagrams of results (see fig. 21, for in- 
stance) by using a crank-arm marked on the 
disc as an index-finger, and recording on the 
base-board the beginnings and ends of the arcs 
swept through by the crank in the various dis- 
tribution-periods. Following upon this, it was 
thought that the eccentric and its rod could be 
eliminated by bringing the disc under the valve 
and fixing upon the valve a rigid slotted arm 
to extend down across the face of the disc and 
engage a pin on the latter, so that rotation of 
the disc would cause reciprocation of the 
slotted arm and the valve. This promised, how- 
ever, to be unmechanical and expensive, and 
was discarded in favor of the model described 
in Chapter I., which comprises the simple 
method of correlating (as described in Chapter 
II.) the movements of valve and crank-shaft 
by scales adjacent to each, so that the reading 
given by the valve on the valve-scale at any 
position of the valve would show what position 
on the eccentric-scale the eccentric ought corre- 
spondingly to occupy. The fact that this in- 
volved a step-by-step adjustment of the valve 
and shaft alternately was found in practice to 
be advantageous rather than the reverse. 



Preface. v 

As to the diagrams and sketches, the writer 
has sought, in some of them, to clear up graph- 
ically some of the incidental minor difficulties 
related to the subject ; see, for instance, figs. 12 
and 13, illustrating " Order of Cranks," fig. 22 
on " Width of Port " ; also compare figs. 23, 
24, and 25, " Double-ported Valve," and see 
fig. $J, which last is in effect a graphical sum- 
mary of results from nearly all the different 
examples which precede it. 

Fig. 36, which concerns the reversal of an 
engine in motion under steam, will probably 
have a special interest to students of the loco- 
motive. 

W. J. T. 



CONTENTS. 



Page 
Introduction to American Edition, i 

CHAPTER I. 
The Simple Slide, 5 

CHAPTER II. 
The Eccentric a Crank. Special Model 

to give Quantitative Results, 13 

CHAPTER III. 
Advance of the Eccentric, 20 

CHAPTER IV. 
Dead Centre. Order of Cranks. Cush- 
ioning and Lead, 24 

CHAPTER V. 
Expansion — Inside and Outside Lap and 
Lead ; Advance affected thereby. 
Compression, 31 

CHAPTER VI. 
Double-ported and Piston Valves, - - 43 

CHAPTER VII. 
The Effect of Alterations to Valve and 

Eccentric, 57 

CHAPTER VIII. 
Note on Link Motions, - ... 66 

CHAPTER IX. 
Note on very early cut-off, and on Re- 
versing Gears in general, 73 



INTRODUCTION TO AMERICAN 
EDITION. 



There is no more important part of the 
steam-engine than the valve — the part which 
determines when and for how long the steam 
shall be admitted to the cylinder, how long it 
shall stay there and how long it shall be al- 
lowed for leaving. And the principles which 
should govern the construction of the valve of 
an engine in order that the steam shall be prop- 
erly admitted and released are the same for 
American, English, French and German en- 
gines. The principles of their construction and 
action are the same, but the details vary. The 
action of every steam-engine depends upon its 
valve, and hence it is important that every en- 
gineer of whatever country should be thor- 
oughly conversant with the principles of the 
construction of valves of different types; 
should know the meaning of " lap," " lead/' 
" advance," etc. ; and should know how a 



2 The Slide Valve. 

change of one of these will affect the others 
and the admission and release of the steam. 
One of the best ways to learn all about valves 
and valve motions is to study them with the 
aid of sectional models, made so that the in- 
side as well as the outside of the parts may be 
seen, but such models are difficult to obtain ex- 
cept at considerable cost. Diagrams, such as 
are so freely used in this work, rank next to 
models in utility and as a means of enabling 
one to thoroughly understand the workings of 
the different parts of machines. The use of 
diagrams requires no knowledge of mathemat- 
ics, and enables explanations to be presented in 
a clear, concise manner. From an educational 
point of view, diagrams possess an advantage 
which models do not, as they make one think 
more. Their use gets one in the habit of form- 
ing mental pictures of the parts of the ma- 
chines discussed, and thus enables one to more 
readily and quickly think out the effect of a 
change in one part on other parts. And this 
ability to picture in the mind the various rela- 
tions of the parts to one another is absolutely 
necessary in order that these relations may be 
thoroughly understood. The man who is ac- 
customed to work out his problems from dia- 
grams and drawings, reasons from cause to 



Introduction. 3 

effect and from effect to cause ; while the man 
who must have a model to work with, works 
on the " cut-and-try " method. 

This little book is confined strictly to an ex- 
planation of the principles which underlie the 
action of the different types of slide valves. 
The plain, simple D-valve, as it is called in this 
country, without lap or lead, is first taken up 
and discussed ; and gradually lap and lead are 
introduced, and the effect of each upon the ad- 
mission, the cut-off, the release, and the com- 
pression is fully worked out and shown. Then 
other types of slide valves are taken up and 
discussed. The advance is made step by step, 
from the most simple form of slide valve to the 
more complicated forms used for attaining cer- 
tain results not so easily attained by the use of 
the simple forms. 

The form of multi-ported valve shown in 
fig. 25 is so much like the Giddings valve used 
on the Russell and some other single-valve en- 
gines, that any one reading the explanation 
given here will have no trouble in understand- 
ing the Giddings valve (shown in fig. 26). 

The multiple admission valve is used to such 
an extent in America that it is but proper that 
a few words should be said of it, and that the 
most common form of multiple admission 



4 The Slide Valve. 

valves should be discussed. And hence the 
Straight Line and the Woodbury valves are 
shown in figs. 27 and 28. 

There are so many engines in this country 
in the front rank of automatic, high-speed en- 
gines which use piston-valves, that a treatise 
on slide valves would be incomplete without 
some mention of this type of slide valve. The 
form of piston-valve shown in fig. 30, in which 
the steam is admitted at the middle and ex- 
hausted at the ends, is well known to all engi- 
neers who have used an Ide or an Ideal engine. 
The valve used on these engines is shown in 

fig. 3 1 - 

Many American engineers prefer engines on 
which the cut-off may be changed without in 
any way affecting the lead, release, or com- 
pression, and, therefore, the description of the 
Meyer valve will be read with interest. Many 
will recognize at once in the valve used on the 
Watertown engine the Meyer valve in almost 
its original form, and in the valve of the Buck- 
eye engine (shown in section in fig. 39), the 
Meyer valve in a modified form. 

J. H. Kinealy, D.E., 

Prof, of Mech. Eng., Washington Univ., 
St. Louis, Mo. 



THE SLIDE VALVE. 



CHAPTER I. 

THE SIMPLE SLIDE. 

The slide valve has always been regarded by 
the writer as the pons asinorum of the student 
of the steam-engine. His own early attempts 
at crossing that bridge were greatly facilitated 
by the simple little device to be presently sub- 
mitted to the reader; with the aid of this ex- 
pedient the student may easily obtain clear 
notions upon all the functions and operations 
of the slide valve, at a nominal cost, and with 
but a small expenditure of time and trouble. 

It is necessary that at the outset the nature 
of the construction above alluded to should be 
clearly appreciated, for the reason that many of 
the explanations given in the succeeding pages 
are referred thereto. 

In passing, it may be remarked that much 
of the difficulty which the ordinary student 



6 The Slide Valve. 

experiences in the study of valves and valve 
gear arises because he approaches the subject 



Nol. 



No 2. 



No 3. 





Fig. i. — Explanatory Diagram. 



with a false impression that it is of necessity 
a difficult one; this idea is generated on the 



The Simple Slide. 7 

one hand in the atmosphere of mystery which 
in the shop is made to surround the matter, 
and on the other hand by the somewhat for- 
midable aspect of most of the geometrical trea- 
tises on the subject. If the beginner can bring 
himself to believe that the slide valve is noth- 
ing more complex than a sliding shutter with 
a cavity in its face, travelling backwards and 
forwards over three ports, the outermost of 
which are alternately opened to steam and 
placed in communication with the central ex- 
haust-port, he will have made a satisfactory 
commencement. 

Take a piece of stout white cardboard and 
upon it set out an enlarged copy of fig. I.* 

The upper portion of this diagram repre- 
sents a section taken at right angles to the 
port-face through a set of steam and exhaust- 
ports as ordinarily arranged ; the graduated 
ring beneath the section will receive considera- 
tion later. 

Postponing for the moment the further in- 
vestigation of fig. i, reference should here be 
made to figs. 2, 3, and 4. 

Fig. 2 shows, in perspective, a cylinder and 
valve-chest with parts removed to make clear 
the manner in which the two steam-ports 

* See also fig. 41. 



8 



The Slide Valve. 



(S^) at the upper ends of the two outer pas- 
sages (PjPjj) afford a communication between 
the valve-chest (VC) and the opposite ends 
(CjCJ of the cylinder (C), and to show the 
passage from the central exhaust-port (EP) to 




FIG. 2.— Perspective Section of Steam-Engine Cylinder. 



the outlet (O) at which the exhaust steam is 
discharged. The relationship between this view 
and the upper part of fig. I will be obvious 
upon a comparison of the two. 

A typical slide valve is shown in section, in 



The Simple Slide. 9 

its place above the ports in fig. 2, and separately 
in figs. 3 and 4. From these latter, the large 
ratio of the width to the length of the valve is 




FlGS. 3 and 4.— Separate Views of the Slide Valve. 

apparent, and it will be seen from fig. 2 that 
the ports are correspondingly made wide and 
short. This arrangement enables a small linear 



io 



The Slide Valve. 



movement of the valve to effect the uncover- 
ing of a large area of port, and to secure free 
passage for the steam with but a small amount 
of valve-travel, and so to reduce to a minimum 
the work unavoidably wasted in overcoming 
friction between the port-face and the valve 
heavily loaded by the pressure of steam upon 
its back. 

Let us return now to fig. i. Draw, near the 
edge of a strip of moderately stiff paper, the < 




Fig. 5.— Section of Slide Valve. 



section of the elementary form of slide valve 
illustrated by fig. 5, taking its dimensions from 
the ports of fig. 1, so that the two views agree 
in the manner indicated. By sliding this dia- 
gram backwards and forwards across the ports 
of fig. 1, the following explanation of the action 
of this, the simplest form of slide valve, will 
be readily appreciated. 

This valve has first to open one port to 



The Simple Slide. 



ii 



steam, and when the steam thus admitted has 
forced the piston towards the opposite end of 
the cylinder, to put the same port in communi- 
cation with the exhaust, while opening the 
other port to the steam which effects the return 
movement of the piston. The duration of the 
admission of steam is the same as that of the 
exhaust, the admission, owing to the dimen- 
sions of the valve in relation to the ports, neces- 




FiG. 6. — Section showing Position of Slide Valve in relation 
to Steam and Exhaust-Ports. 



sarily taking place on one side of the piston 
simultaneously with the occurrence of exhaust 
from the other side (see fig. 6). The travel 
of this rudimentary valve equals twice the 
amount to which the port is opened to steam. 
It happens to open each port fully in this case 
(although this is rarely the case with modern 
slide valves), so that the " travel " equals twice 



12 The Slide Valve. 

the width of steam-port: twice, because (start- 
ing with the valve in its central position, as 
shown in fig. 5) it has to move to the right, 
say, by an amount equal to the width of the 
left-hand port, in order to open that port fully 
to steam, and then it moves back again and 
travels to the other side of the central position 
by an equal amount, in order to open the 
equally dimensioned right-hand port to steam. 
In fig. 6 the valve is shown at the two extremes 
of its travel, in full lines at one end, and in 
dotted lines at the other end. 



CHAPTER II. 

THE ECCENTRIC A CRANK. SPECIAL MODEL TO 
GIVE QUANTITATIVE RESULTS. 

The necessary travel of the valve is given to 
it by means of an eccentric, which is keyed 
upon or formed as part of the crank-shaft. The 
eccentric is, in effect, a crank, whose throw 
equals the amount of eccentricity of the sheave. 
This may not be obvious ; let us investigate a 
little. Suppose that we have a big crank-shaft, 
and want to put a little crank in the middle of 
it (for this is what happens in the case of a 
steam-engine, the travel of the v alve being such 
that a crank of the usual I ] form 

would be disproportionate to the shaft of which 
it formed part — like fig. 7, perhaps). Now. 
that such a shaft would be extremely weak at 
the pin goes without saying. Imagine, that 
to lessen the weakness the crank-pin of this 
little crank is made of greater diameter, as in 
fig. 8, or even more so, as in fig. 9 ; still we get 
the same result, which is that the throw of the 



14 



The Slide Valve. 



crank remains the same, and equals the dis- 
tance of the centre of the crank-pin from the 
centre of the crank-shaft — i.e., the amount that 
this crank-pin is u ex-centric" out of centre, 




i 

i 
i 

ill ~M 
i p 


II ! 1 



r 



—t 



Fig. 7 .— Half-travel of Valve. 



and the eccentric sheave being simply an ex- 
aggerated crank-pin we get back to our origi- 
nal statement that it is virtually a crank whose 
throw equals the half-travel of the valve, which 



The Eccentric of a Crank. 



15 



in turn equals the amount of eccentricity of 
the sheave. 

Being fixed to the crank-shaft and moving 
with it, always in a fixed relationship to the 




f-Jb-H 



Fig. 8.— Half-travel of Valve. 



main crank, the eccentric operates the valve 
in the necessary accordance with the movement 
of the crank and piston, as will presently be 
explained. 



i6 



The Slide Valve. 



Returning now to fig. 5, it must be under- 
stood that a valve of the type exemplified 




Fig. 9 —Half-travel of Valve. 



therein must be in the middle of its travel 
whenever the piston is at either end of its 
stroke. The reason for this must be clearly 



The Eccentric of a Crank. ij 

appreciated for the sake of what follows here- 
after ; it may be arrived at very easily with the 
aid of fig. i. Put a disc of card in the circle 
provided in that figure, and let it turn about 
a drawing-pin stuck through its centre ; this 
disc is the equivalent of a crank-shaft. A 
crank-arm (C) may be permanently marked 
upon it in ink, as in fig. 10 ; another arm, which 
may be marked upon it in pencil, will serve 
to represent a " valve-crank/' the equivalent 
of the eccentric by which the valve is to be 
operated. 

Actual connection between that valve-crank 
and the valve being dispensed with, some 
means for making the movement of the valve 
correspond accurately with that of its crank 
must be provided. This provision is made by 
the numbered scales * in fig. i ; the circular 
scale within which the card disc revolves is 
numbered in correspondence with scales on the 
ports, and to these latter an arrow on the strip 

* The horizontal scales on the port-faces are obtained by 
projecting up equidistant circumferential marks from various 
circles having different valve-cranks for their radii. The scale 
(No. i ) is for the elementary form of valve, without lap ; it serves 
also for link-motion in mid-gear ; No. 2 is for a valve having 
outside lap ; and No 3 is for link-motion "linked up." The 
student need not, unless he pleases, troub e himself to investi- 
gate the construction of the diagram, which is drawn to scale, 
so that if copied accurately it will serve to give him the dem 
onstrations desired. 



i8 



The Slide Valve. 



of paper which carries the valve is adjusted 
(see also fig. 10). To find the position in 
which to mark the arrow on the valve, put the 
latter in the centre of its travel and mark the 




arrow-head just above the " O " on the No. i 
scale. When the disc is turned, the end of 
the eccentric-arm or valve-crank travels with 
it around the circular scale. When the eccen- 



Model to give Quantitative Results. 19 

trie-arm is moved step by step around the 
circle to the various numbered graduations, the 
valve must be moved step by step to bring its 
arrow-head to the corresponding numbers on 
the horizontal scale on the ports. Thus the ec- 
centric and valve, although not moved simulta- 
neously, as they would be if a rod connected 
them, are kept in accurate relationship whilst 
moved independently. If the reader will now 
read the preface, which he has probably 
skipped, he will the more readily appreciate 
the principle of this arrangement. 

Further, taking the main crank-arm (C) as 
an index-finger, the position of its end at the 
points of cut-off, release, etc., with different 
valves and eccentrics, may be marked in pencil 
on the circular scale, or better, on a piece of 
tracing-paper interposed between the disc and 
that scale; by this. means the student will ob- 
tain circular diagrams enabling him to compare 
the results due to different settings and pro- 
portions of gear, and he will also discover in 
what manner those results are affected by indi- 
vidual elements of the mechanism in any given 
example. 



CHAPTER III. 

ADVANCE OF THE ECCENTRIC. 

Assume a horizontal engine, with the main 
crank moved into a horizontal position so that 
the piston would be at, say, the left-hand end 
of its stroke. Assume also that the engine is 
at rest, and put the valve in the centre of its 
travel (as in fig. 5), when its faces exactly 
cover the steam-ports and its cavity covers 
the bridges and the exhaust-port; the first 
movement of the valve is wanted to be to the 
right, in order to admit steam to the left-hand 
end of the cylinder to move the piston also to 
the right, no matter whether the crank-shaft 
is to have negative or positive rotation. Now, 
a valve of this pattern evidently will not admit 
steam to the cylinder to start the movement 
of the piston (supposing that the engine has 
not yet been started), wdiich will therefore 
have to be helped by some external agency; 
admitting this as a point which shall be touched 
again presently, place the crank next, so that 



Advance of the Eccentric. 21 

the piston would be at the opposite end of its 
stroke; the valve, which had to move to the 
right as explained, must have come back into 
the position whence it started, and be in readi- 
ness to move equally to the left, afterwards 
returning once more to the central position 
by the time that the piston gets back to the 
left-hand end of the cylinder. Hence we see 
that, whenever the piston is at either end of its 
stroke, this valve must be in the middle of its 
travel. 

If this be granted, we may proceed further: 
— Having the piston at the right-hand end of 
its stroke, for instance, draw a line on the disc 
at right angles with the crank (see fig. 10) ; 
this will represent two " eccentric-arms " of 
the necessary throw if each arm equals in 
length the half-travel of the valve ; either of 
them at present suits the position of the valve, 
which is in the middle of its travel. Choose, 
now, the direction in which your crank-shaft 
shall rotate; let us suppose, for example, that 
it shall have " right-handed " rotation, and 
place an arrow on the disc to indicate* the di- 
rection of rotation. Under these conditions 
the upper eccentric will follow the crank round, 
operating the valve to close the right-hand port 
and open the left-hand one, admitting steam to 



2.2 The Slide Valve. 

stop the engine. Cross this upper arm out, 
then, for manifestly we have to use the lower 
eccentric, which, going ahead of the crank, 
will cause the valve to open the right-hand 
port when required. But now conversely, 
suppose that the engine is to be run in the 
opposite direction; the upper eccentric (repre- 
sented by the crossed-out line) now goes ahead 
of the crank, and gives the valve motion in 
the right direction, consequently the lower 
one must now go out of use, for under the 
altered conditions of working it tends to move 
the valve in the wrong direction. The deduc- 
tion from this reasoning is, that in whichever 
direction an engine runs, the eccentric used for 
the time being must be set in advance of the 
crank, and that advance must be at least po°. 

If it were less than 90 the wrong port 
would be opened — the port at the opposite end 
of the cylinder to that at which the piston 
might happen to be. This effect may be shown 
by purposely giving the eccentric less advance. 

The amount which the angle between the 
eccentric and the crank exceeds 90 is the 
" angle of advance " ; and the distance which 
the valve is moved from mid-position when the 
piston is at the end of the stroke is the " linear 
advance." 



Advance of the Eccentric. 23 

Because of the necessity for different set- 
tings of the eccentric for different directions of 
rotation of the crank, engines required to be 
reversible usually have either : 

(1) Means for shifting the position of a 

single eccentric upon the shaft, that 
it may always be placed so as to lead 
the crank. Or 

(2) Two eccentrics fixed upon the crank- 

shaft so that one of them is always 
ahead of the crank in whichever di- 
rection it rotates, mechanism being 
arranged in connection with the ec- 
centrics, so that the valve can be 
driven by the leading eccentric, or 
operated by the conjoint action of 
both eccentrics. Or 

(3) Radial or other special valve gears. 

The simple form of slide valve shown in 
fig. 10 does not permit of the economical use 
of steam, inasmuch as it allows steam at full 
pressure to follow the piston for the whole of 
the stroke, and does not admit of the use of 
its expansive properties, for the simple reason 
that, at the instant the admission of steam 
ceases, the exhaust of the same body of steam 
must immediately commence, as may be clearly 
seen from the circular diagram. 



CHAPTER IV. 

DEAD CENTRE. ORDER OF CRANKS. CUSHION- 
ING AND LEAD. 

It ivS desirable at this point to clear the ground 
by touching for a moment upon one or two 
elementary matters which, if not now ex- 
plained, might cause confusion subsequently. 

Dead Centres. — A crank is said to be " on 
the centre " or " on the dead centre " when the 
connecting-rod and crank are in line, and this 
occurs twice in every revolution, as shown in 
fig. ii. 

Right-hand Crank to Lead. — The phrases 
" right-hand crank to lead," or " left-hand 
crank to lead," are sometimes used, and are 
confusing to beginners ; the " leading " crank- 
is that one which leads when the engine is run- 
ning ahead. In most two-crank engines one 
of the cranks will be a quarter of a revolution 
(or less than half a revolution, where the 
cranks are not set at 90 degs.) in advance of 
its neighbour, and will, therefore, lead it in the 
direction in which it should go. Of course, 



26 



The Slide Valve, 



that neighbour might be said to lead the other 
by being three-quarters of a revolution in ad- 
vance (or more than half a revolution, where 
the cranks are not set at 90 degs.), and this is 
where a little confusion sometimes arises ; the 
leading crank must always be taken as that 
one which is less than half a revolution in ad- 
vance of its fellow when the engine is running 



RIGHT HAND 
CRANK LEA.OINC 



LEFT HAND 
CRANK LEADINQ 




AHEAD 
FIG. 12.— Order of Cranks. 



ahead, and in locomotives and similar engines 
is " left-hand " or " right-hand/' according as 
it lies to the left or right of a spectator looking 
from behind the crank-shaft towards the cyl- 
inders. (See fig. 12, which shows two arrange- 
ments of the driving cranks of an English loco- 
motive. The cylinders are supposed to be to 
the right of the wheels.) 



Order of Cranks. 27 

The accompanying diagram (fig. 13) will per- 
haps serve to make clear without further ex- 
planation the meaning of the expression, used 
with reference to three-crank marine engines, 
of " order of cranks, high, intermediate, low," 
or " crder of cranks, high, low, intermediate/ 1 
as the case may be. 





Fig. 13.— Order of Cranks. 

Let us revert here to one of the points pre- 
viously noted, which was that the simple valve, 
when we had it in connection with an eccentric 
set 90 degs. in advance of the crank, would not 
admit steam to the cylinder at the exact com- 
mencement of the stroke, i.e., when the crank 
is on either of the " dead centres." 



28 The Slide Valve. 

Keeping this fact in mind, we will consider 
the matters of 

Cushioning and Lead. — It has long beer- 
found desirable (especially in quick-running 
engines) that the motion of the returning pis- 
ton should be opposed as it arrives at either 
end of its stroke, so that the moving weights 
of piston, piston-rod, and connecting-rod may 
be " cushioned," to prevent injurious stresses 
and to conduce to easy running. To use a 
homely illustration, one might say that just in 
the same way is it desirable, when striking out 
from the shoulder, to have one's adversary 
within range, that he may provide the neces- 
sary " cushioning," and so prevent stresses in 
one's arm and possible dislocations. 

This " cushioning " is provided in part by 
checking the exit of the exhaust steam, as will 
be shown later, and in part by setting the ec- 
centric a little more than 90 degs. (fig. 14) in 
advance of the crank, so that the valve com- 
mences to open the port to admit steam in 
front of the returning piston before it arrives 
at the end of its stroke, continuing to open 
the port as the piston comes to a rest, and is 
started on the return stroke. 

The amount to which the port is found to be 
open when the piston is at the end of its stroke 



Cushioning and Lead. 



29 



is called the " lead " of the valve, and it would 
be expressed as " one-eighth of an inch lead," 
or " three-sixteenths of an inch lead," etc., 




as the case might be. To set this valve for 
lead, keep the piston at the end of its stroke, 
and set the eccentric back until the valve is 
opened to the desired lead ; then the eccentric 



30 The Slide Valve. 

would be fixed in its newly-found position, as 
shown in fig. 14. The radius of the " eccen- 
tric-arm/' which is the lighter of the two 
radial lines on the disc in fig. 14, is equal 
in length to one-half of the travel of the valve, 
and therefore does not reach the edge of the 
disc, at which, however, there is a short guid- 
ing-mark in the line of the eccentric-arm 
" produced." By using this guiding-mark and 
moving the arrow-head of the valve over the 
scale on the ports, in correspondence with the 
movement of the guiding-mark on the edge of 
the disc within the circular scale, the valve, 
the main crank (C), and the eccentric can be 
operated in concord, very much as if they were 
mechanically connected. 

The explanation just given of the meaning 
of lead, and of its effect, should be borne in 
mind, as we shall presently have to consider it 
in conjunction with other matters, for, as has 
been said, there is something more than 
" lead " employed in the production of cush- 
ioning. 



CHAPTER V. 

EXPANSION INSIDE AND OUTSIDE LAP AND 

lead; ADVANCE AFFECTED THEREBY 

COMPRESSION. 

Expansion. — In order that the expansive prop- 
erties of steam may be utilised, it must be cut 
off before the piston arrives at the end of its 
stroke, and on being cut off must not be al- 
lowed to commence exhausting immediately, 
but must be confined in the cylinder, in order 
that by expanding it may force the piston 
further before it. The old valve, as we have 
seen it, did not allow this, and to effect it the 
faces of the valve are lengthened, so that after 
the steam is cut off by the edge (a) of the 
valve (fig. 15) the steam is confined, and ex- 
pands in the cylinder until the edge(&) arrives 
at c, when exhaust immediately commences. 
Now put the piston at the end of its stroke, 
and put the new valve in a central position 
(fig. 16). The distance that the valve, when 
in this central position, overlaps the outer edge 



32 



The Slide Valve. 



of the steam-port, is called the " outside or 
steam lap/' or more commonly the " lap " of 
the valve, that is, the distance d a (fig. 16). 

The first thing noticeable is that this valve 
must be given considerably more travel than 
the old one, as, to open fully each port to 
steam, we have to bring the edge a to c, first 
for one end of the valve and then for the other, 




Fig. 15. 



so that the travel of this valve must be twice 
the distance from a to c — i.e. (lap plus port- 
opening) X 2, whereas for the old valve it 
was the distance from d to c which had to be 
multiplied by two to give the travel. 

Increased travel means increased throw of 
the eccentric, and we draw on the disc in 
fig. 16 two longer .eccentric-arms' than before 



Outside Lap. 



33 



(the bolder dotted lines) for the forward and 
backward eccentrics. We want " lead " with 
this valve, and we must therefore give each 
eccentric more advance than the 90 degs. 





Fig. 16. 



shown. Now with the old valve we give each 
eccentric an advance upon the 90 degs. which 
was determined solely by the lead, no lap being 
then in question ; but we now have a lap to 
deal with, therefore each eccentric must be 
set on past 90 degs. until it first has drawn 



34 The Slide Valve. 

edge a of the lap to the outer edge of the port, 
and then still further until the edge a gives 
the necessary " lead/' Hence we see that the 
linear advance of the eccentric = lap phis lead, 
so as to draw the valve out of its central posi- 
tion by that amount. 

Travel, then, depends upon outside lap and 
port-opening, and lead does not affect it. 

Advance depends upon lap and lead. 

The advance of the eccentric is usually 
stated by specifying the angle of advance or 
the number of degrees exceeding 90 degs. that 
the eccentric is in advance of the crank — for 
instance, " an angle of advance of 10 degs." 
means 90 degs. -f- 10 degs. = 100 degs. in ad- 
vance of the crank. 

The line of travel of the valve is sometimes 
inclined to the line of travel of the piston; in 
such cases, one of which is illustrated in fig. 17, 
the angle of advance must, of course, be meas- 
ured from a line at right angles to the valve- 
travel line, and not at right angles to the line 
of piston-travel, as we have hitherto assumed. 

We must now carefully consider the further 
results obtained by the use of a valve having 
lap and giving lead, and whose eccentric of 
greater throw is advanced accordingly. 

We should expect cut-off to take place 



36 



The Slide Valve. 



earlier than with the old valve, for the eccen- 
tric is advanced, and the edge a (fig. 15) 
would arrive at e a little earlier than would the 
edge d of the old valve, even without advance 
of the new eccentric. 

This seen, we can understand that in caus- 
ing the edge a (fig. 18) to reach the edge e 




Fig. 18. 



earlier in the stroke of the piston than it 
otherwise would do, we shall have hastened 
all the other, operations so that they must 
occur earlier in the stroke (not necessarily by 
exactly the same amount for each, however, 
as we shall see further on), hence the edge b 
(fig. 18) arriving earlier at c, exhaust will 
commence before the piston completes its 



Advance and Lap, 



37 



stroke, and also in returning to the right 
(fig. 19) the valve is earlier, and instead of 
the exhaust lasting during the whole of the 
return stroke some of it will be shut in by b 
and compressed before the returning piston 
until the edge a arrives at e, whereupon the 
lead commences coming in on and reinforcing 




the compressed steam. Thus it is that com- 
pression and lead together affect the cushion- 
ing, and consequently the piston is eased in 
stopping, and the return stroke is commenced 
without injurious shock or jar. A similar 
operation, of course, takes place at the opposite 
end of the cylinder when the piston arrives 
there. 



38 



The Slide Valve. 



The distribution-diagram for a valve with 
lap, lead, and a suitable eccentric is given in 
fig. 20, which should be compared carefully 
with figs. 10 and 14. 

Inside Lap, exhaust lap, or " cover " as it is 




Fig. 20. 



sometimes called, is possessed by many slide 
valves, and it is the amount that the valve 
when in its central position overlaps the inner 
edges of the steam-ports. Conversely, 

Inside Lead is the amount of space left, in 
certain cases, between the inner edge of the 



Inside Lap and Lead. 39 

valve and the inner edge of the steam-port 
when the valve is in its central position. 

The amount of inside lap does not affect the 
setting of the eccentric, neither does it necessi- 
tate any alteration in the travel of the valve. 
With a given cut-off, steam is confined longer 
in the cylinder by a valve having inside lap 
because of the lengthening of the valve-face 
due to the inside lap, and compression will 
commence earlier and will be of longer dura- 
tion from the same cause ; exhaust being 
shortened in two ways — by the amount added 
to the duration of expansion at its end, and 
by a similar amount added to the compression 
at the commencement thereof. 

Inside lead may be considered as something 
taken off the exhaust edge of the valve, and. 
in effect, exactly the opposite to exhaust lap, 
for, as may easily be seen, it shortens expan- 
sion and compression by reason of the short- 
ening of the valve-face, and lengthens the 
exhaust at commencement and end by the 
amounts by which the operations of expansion 
and compression are shortened. The left-hand 
side of the valve in fig. 21 has inside lap, whilst 
the right-hand side has inside lead, and the 
circular distribution-diagram below compares 
the distribution in the case of valves having 



40 



The Slide Valve. 



inside lap and inside lead with the distribution 
for an ordinary valve such as was considered 
with reference to fig. 20. By reference to the 







$ 






Fig. ax. 



lower portion of fig. 21, it may be seen that 
the operations of admission and cut-off are 
unaffected, as they are performed by the outer 
edges of the valve ; but 



Inside Lap and Lead. 



4i 



r is shorter and exhaust earlier 
with inside lead. 



xpansion ^ j s j on g er anc [ ex haust later 
I with inside lap. 

is later and shorter with 

inside lead. 
Compression + - s earlier and longer with 

1^ inside lap. 
From this it will be obvious why many ex- 



4 




OF VALVE 



.MAXIMUM. J 
- 0PEN9 \ 




Fig. 22. 



press locomotives are given inside lead, and 
why many good engines have inside lap. 

Free Exhaust.— In order to give as free a 
passage as possible to the exhaust steam, so 
that it may leave the cylinder easily, and not 
have to be forced out by the returning piston 
and cause back pressure thereupon, it is cus- 
tomary to make the steam-ports wider than 



42 The Slide Valve. 

the amount to which they are open to steam. 
This must on no account be confused with 
inside lead, which is quite a distinct matter. 
The effect of having wide ports is that the 
exhaust may be opened to any extent up to the 
half-travel of the valve, although the steam- 
opening may be much smaller, as may be 
understood from the exaggerated diagram 
(fig. 22), whence also should be seen that this 
widening of the port does not affect the dura- 
tion of expansion or of any other operation 
of the valve, neither does it necessitate any 
allowance in the setting of the eccentric. It 
simply provides a free exhaust. 



CHAPTER VI. 

DOUBLE-PORTED, MULTIPLE ADMISSION, AND 
PISTON VALVES. 

Double-ported Valves. — Tn large marine en- 
gines using considerable quantities of steam 
per stroke, and consequently requiring a large 
amount of port-opening, the ordinary slide 
valve would require an undue amount of 
travel. To obviate this, each end of the cylin- 
der may have two steam-ports, both ports 
opening into one passage as shown, and a 
double-ported valve. The double-ported valve 
is virtually a combination of two slide valves 
in a single casting (fig. 23), one valve for the 
inner set of steam-ports and the exhaust-port, 
the other valve being halved, as it were, for 
each outer steam-port. The exhaust cavity of 
each of the outer " half-valves " having a pas- 
sage leading to the exhaust cavity of the inner 
valve, both the valves exhaust into the central 
port, which is widened accordingly. The 
fig. 23 is merely diagrammatic, to enable the 



44 The Slide Valve. 

principle of this valve to be easily borne in 
mind; the next drawing (fig. 24) shows in 
section a double-ported valve as actually con- 
structed, with the exception that the valve- 
spindle and its chamber are omitted in order 
that the form of the valve may be more clearly 
shown. The section is taken in two planes, 
one upon the longitudinal centre line of the 
valve, and the other (the left-hand half of 
the valve) a little to the near side of the centre 




FIG. 23.— Diagram showing Flow of Steam and Exhaust 
through a Double-ported Slide Valve. 



line. Fig. 25 shows a sectional view of a 
double-ported valve in perspective. 

Now, any movement given to the " inner 
valve " is given, of course, to the outer one, 
so that if the inner valve be moved so as to 
give, say, 2 ins. of port-opening, the outer 
valve gives at the same time another 2 ins. of 
port-opening; therefore, with a given move- 
ment of a double-ported valve we get double 
the amount of port-opening that we should 



Double-ported Slide Valves. 



45 



have obtained with the ordinary valve. Hence 
the travel of the double-ported valve = (lap 
+ half the two port-openings) X 2; whereas 
that of the ordinary valve = (lap -f- the port- 
opening required) X 2. 

The inner and outer valves of the double- 
ported construction, acting simultaneously, 
have each the same amount of lap ; but they 
give the lead between them, and the eccentric 
is set with a linear advance equal to one of the 




Fig. 24.— Vertical Section through a Double-ported 
Slide Valve. 



laps and one of the lead-openings. The dis- 
tribution of steam with this valve is similar 
to that of the ordinary valve whose distribu- 
tion-diagram was shown in fig. 20. The dif- 
ferent periods are of the same duration in the 
example chosen, but the operations of cutting 
off steam and of opening exhaust are per- 
formed by the edges of the inner and outer 
valves instead of by a single edge as in the 
ordinary valve. 



46 The Slide Valve. 

A valve very similar to the double-ported 
valve, shown in fig. 23, is the Giddings valve 
used on the Russell engines and some other 
automatic high-speed engines in America. By 
its peculiar construction, this valve obviates 
the necessity of double steam ports, yet it is so 
much like the double-ported valve that it may 
be noticed here rather than under the head of 
multiple admission valves, where it might be 
said to properly belong. This valve accom- 




FlG. 25.— Perspective view (in Section) of a Double-ported 
Slide Valve. 

plishes exactly what the double-ported valve 
does, and in pretty much the same way. The 
Giddings valve in fig. 26 shows steam being 
admitted to the right-hand end of the cylinder 
and exhausted from the left-hand end. The 
travel of this valve is, as in the case of the 
double-ported valve = (lap + half the port 
opening required) X 2. The valve should be 
set with a linear advance equal to the lap plus 
one-half the required lead. 



Multiple Admission Valves. 

MULTIPLE ADMISSION VALVES. 



47 



In America, the type of engine known as the 
automatic high-speed engine is very much 
used. These engines have a wide range of cut- 
off, from almost o to J stroke, and in order to 
avoid a long valve and a long travel and to 
secure a quick admission and cut-off of the 
steam, recourse is had to multiple admission 




Fig. 26.— The Giddings Valve. 

valves. These valves are so constructed that 
when one of them is moved from midposition, a 
distance equal to, say, 1-16 of an inch more than 
the steam lap, the width of the opening for the 
steam to pass through into the cylinder is 1-16 
multiplied by the number of admissions of the 
valve. Thus, if the valve is a double admis- 
sion valve, and it is moved 1-16 plus the steam 



48 



The Slide Valve. 



lap from midposition, the port is opened a dis- 
tance equal to 2 X 1-16 = ■§ of an inch. 
Double admission valves are quite common 
and there are also a number of quadruple ad- 
mission valves used. They are made as flat 
valves and as piston valves, but all are made 
with a common object in view, that is, to se- 
cure an early cut-off and quick opening and 
closing of the ports with a valve whose length 
and travel shall be short. 




Fig. 27 —The Straight Line Valve. 



One of the best-known forms of double ad- 
mission valves is the " Straight Line " or 
" Sweet " valve, which was invented by Pro- 
fessor John E. Sweet, formerly of Cornell Uni- 
versity, and first used by him on the Straight 
Line engines. This valve is now used by a 
large number of manufacturers of automatic 
high-speed engines. The valve is shown in 



Multiple Admission Valves. 49 

section in fig. 27, where a is the " cover plate," 
and b is the valve, proper. The valve, b, moves 
back and forth between the cover plate, a, and 
the valve seat. The cover plate rests on pieces 
called " distance pieces," placed at the sides of 
the valve. These distance pieces are made of 
such a thickness that while the cover plate can- 
not touch the valve, it is so close to it as to 
make a steam tight joint between the plate and 
the valve. Therefore, the valve acts as a piston 
of rectangular cross-section, moving back and 
forth between the cover plate and the valve 
seat. The result of its peculiar construction is 
that a balanced flat valve is secured ; a valve 
which works with very little friction, as there 
is no surface upon which the steam can act so 
as to press the valve against the valve seat. 
The cover plate is kept in place by the pressure 
of the steam against its back, aided by one or 
two small springs inserted between the plate 
and the steam-chest cover. 

This valve should be set so that the distance 
the valve is from midposition when the piston 
is at the end of its stroke, or the linear advance, 
shall be equal to the steam lap plus one-half 
the desired lead. The travel of the valve is 
equal to twice the sum of the steam lap and 
one-half the maximum opening of the port for 



50 



The Slide Valve. 



steam, or 2 X (steam lap + \ maximum open- 
ing of port for steam). 

In the case of a quadruple admission valve, 
such as the Woodbury valve shown in fig. 28, 
the travel is equal to twice the sum of the steam 
lap and one-fourth the maximum opening of 
the port for steam, or 2 X (steam lap + i 
maximum opening of port for steam). 

The different phases of the movement of a 




Fig. 28.— The Woodbury Valve. 



multiple admission valve are just the same as 
for the ordinary slide valve, the only difference 
being in the suddenness with which the ports 
are opened and closed. 

Piston Valves. — It may be readily seen that 
between the ordinary flat slide valve and the 
valve-seat considerable friction is set up, be- 
cause the steam pressure on the back of the 



Piston Valves. 51 

valve is only partly balanced by the varying 
pressure in the cylinder steam-passages acting 
upon small portions of the. area of the 4t rub- 
bing " or under-side of the valve ; but by mak- 
ing the port-faces cylindrical and making the 
faces of the valve in the form of pistons, the 
pressure is given no surface upon which to 
press in the direction of the valve-seat ; or, in 
the case of hollow piston valves (an example 
of which will be presently illustrated), al- 
though the steam has access to the interior of 
the tube, pressure on parts thereof is balanced 
by an equal pressure in an opposite direction 
upon exactly opposite parts, and therefore is 
confined to stressing the valve and has no ten- 
dency to force any part of it on to the valve- 
seat. 

In the plain piston valve shown (fig. 29) 
the faces are formed by the two valve pistons, 
which work in a cylinder wherein steam and 
exhaust openings are made. 

Sometimes the steam-supply enters between 
the two pistons, as in figs. 30 and 31, exhaust 
taking place into the spaces at the ends of the 
valve, which spaces are connected by passages 
formed in the valve, which is tubular, as 
shown. In such valves the steam-lap must be 
put upon the inner edges of the valve pistons, 



5* 



The Slide Valve. 



for the steam and exhaust edges have their 
positions reversed ; and the lead will also be 
given by the inner edges. 

The eccentrics for a valve with internal 




^77777777777777777777, 



Fig. 



steam-supply must be placed 180 degs. in 
advance of the positions which they would 
occupy if the valve were of similar propor- 
tions, but with external steam-supply, for the 
reason that the internally supplied valve must 



Double-ported and Piston Valves. 53 

be moving up, for instance, when the ordinary 
valve would be moving down, and vice versa. 

If the valve be an externally supplied valve, 
but driven from one end of a two-armed 
rocking-shaft, the valve gear actuating the 
other end, as in American locomotive practice, 
the eccentrics must, for obvious reasons, be 
set 180 degs. in advance of their usual posi- 
tion. 

Fig. 30 is given by the kind permission of 
Messrs. W.Denny & Company, the well-known 
Clyde ship-builders, and it possesses many 
points of interest, one of which is that the 
packing in the valve-spindle stuffing-box below 
the valve, and not shown in the drawing, is 
subject only to the influence of the tempera- 
ture and pressure of exhaust steam, instead 
of to that of the higher temperature and press- 
ure of the supply steam ; this is a matter of 
importance, having in view the high pressures 
of steam which are now used. 

Further, it may be seen that the upper pis- 
ton of the valve is of greater area than the 
lower one, and that the high-pressure steam 
entering between the two pistons will conse- 
quently exert a total pressure upward upon 
the larger piston greater in amount than the 
downward pressure exerted upon the low T er 



54 



The Slide Valve. 



and smaller one ; thus the valve has a tendency 
to rise, so long as steam is on. This upward 
tendency balances wholly or in part the down- 




ward tendency due to the weight of the valve, 
which has not to be lifted by the valve gear, 
which may be of a lighter character than 
usual, and give less trouble in working, owing 



Double-ported and Piston Valves. 55 

to diminution of wear and tear. The valve- 
spindle of the ordinary flat valve is sometimes 
prolonged (where the valve works vertically) 
to end in a small piston exposed to steam 
pressure, and acting so as to balance the 
weight of the valve. 

In the " Joy " assistant-cylinder, a balance- 
piston is provided with special passages which, 




Fig. 31.— The Ide Valve. 



in the movement of the valve-spindle, are 
brought opposite ports in the assistant-cylinder 
wall, so that steam is let in to propel the 
balance-piston up or down, and thereby assist 
the valve gear in each stroke. The balance- 
piston acts as its own exhaust valve in sliding 
over exhaust-ports in the assistant-cylinder 
wall. 



56 The Slide Valve. 

Fig. 31 shows the piston valve used in 
America on the well-known Ide and Ideal en- 
gines. It is very similar to the valve shown in 
fig. 30, and has the same advantages which 
have been pointed out as belonging to that 
valve. As, however, it is principally used on 
horizontal engines, the two pistons are made 
of the same diameter, in order to secure a per- 
fectly balanced valve. 



CHAPTER VII. 

THE EFFECT OF ALTERATIONS TO VALVE AND 
ECCENTRIC. 

Advance of the Eccentric. — It should here be 
established that to any given example of valve, 
ports, and eccentric (leaving the piston out 
of consideration for a moment) belongs a cer- 
tain cycle of operations, the occurrence of these 
operations and their duration, expressed in 
fractions of a revolution of the rotating eccen- 
tric, being constant for a given case/' but the 
fraction of stroke traversed by the piston 
between or during any of these operations 
of the valve will depend upon where in the 
stroke of the piston the cycle of operations is 
started, taking any convenient operation as 

* To make the idea clear one might plot these operations 
round a circle representing one revolution of the eccentric, 
when it would be seen that the operations of the valve could 
be considered quite independently of those of the piston — then 
it would i emain to show what the piston might do, or have done 
upon it, by comparing its motion under varying conditions 
with the U7ivaryi?ig operations of the valve, in the manner ex- 
plained later. 



58 The Slide Valve. 

being the commencement of the cycle, for the 
speed of the piston rises from nil at the com- 
mencement of its stroke to a maximum near 
the middle, falling again to nil at the end. 

Now we may go on to consider the effect 
of increasing the advance of an eccentric be- 
yond that which it would ordinarily have 
(fig. 32). If we suppose our eccentric and 
valve to give us a cut-off at C and an exhaust 
at E, expansion lasting 0.17 of the stroke (as 
shown in full lines, fig. 32), and that by ad- 
vancing the eccentric we get a cut-off at C 1 
0.14 earlier, we must not expect to find that 
exhaust also will be 0.14 earlier (dotted lines, 
fig. 32), for although the exhaust edge of the 
valve always gives exhaust at a certain con- 
stant period of time later than steam is cut 
off by the steam edge, yet in that interval the 
speed of the piston slackens less in this sec- 
ond case than in the first example, and it 
therefore travels through a greater distance 
than in the same time-interval taken later in 
the stroke in the first case. Hence, with in- 
creased advance we get earlier cut-off, with 
expansion of lengthened duration because ex- 
haust is not earlier in the same degree, neither 
are all the other operations performed by the 
valve, although they each commence earlier 



Advance of the Eccentric. 



59 



and last longer in the stroke, lead included, 
but admission, of course, excepted. The idea 
may, perhaps, be more easily grasped by keep- 




<r 



061 QJS 

•STROKE 

loo C-1L*L 

Cl El 

Fig. 32. 






ing the eccentric and valve in view at an un- 
altering standard of reference, and when it is 
desired to consider the effect of advancing the 



6o The Slide Valve. 

eccentric, to imagine instead that it is the set- 
ting of the main crank which is altered — 
which in effect is the same thing, and is, per- 
haps, easier to reason out. 

Lengthening or Shortening the Eccentric 
Rod — Shifting the Valve on its Spindle. — 
Starting with the valve in its central position, 
and lengthening or shortening the eccentric 
rod or shifting the valve on its spindle, or 
combining both operations in the same sense, 
the distance between the centre of the crank- 
shaft and the centre of the valve is altered, 
and the travel of the valve is shifted bodily to 
the right or left, i.e., the valve when in the 
centre of its travel (the extent of travel re- 
maining of course unaltered) will be either 
further from or nearer to the crank-shaft than 
before, while the position of the ports remains 
unaltered ; therefore the arrow upon the valve 
at its centre (see fig. 16) lies to the right or 
left of the ^ centre line of the port-face, and, 
that arrow being no longer appropriate as a 
pointer at mid-travel to the zero of the scale 
on the ports, we must adopt as our index- 
finger the graduation mark made adjacent to 
the arrow and lying over the said zero, in 
order that when the disc may indicate that 
the valve should move, say, from o to 6 the 



Shifting a Valve 61 

new mark or index-finger, and not the arrow, 
may be given that movement. 

If the valve had previously been properly 
set for lead, and the alteration shifted it | in. 
to the left, for example, we should find that 
the lead at the left-hand end would be dimin- 
ished by that amount, and that subsequently, 
at cut-off, release, and compression, for both 
ends of the cylinder, the valve would always 
be found \ in. to the left of its proper position. 
The result would be, as shown in fig. 33, that : 
At left-hand of cylinder (see lines of dashes) : — 

^ earlier, these opera- 
1 tions taking place 
as the valve moves 
tozvards the left. 
Compression commences later, this opera- 
tion taking place as the valve moves from 
the left. 
At right-hand end of cylinder (see lines of 
dots) :— 

_ ' . ^1 later, because when 

Lead increases ~ ,. .. ,, 

_ . I effecting them the 

Exhaust commences r 



Lead decreases 
Cut-off takes place 
Exhaust commences 



Cut-off takes place 



valve is moving 
from the left. 



Compression commences earlier, for it oc- 
curs whilst the valve is moving to the left. 



Shifting a Valve. 63 

The alterations at opposite ends being in oppo- 
site sense. (The normal distribution is shown 
by full lines for comparison.) And although 
the commencement of the operations is ad- 
vanced and retarded by the same amount of 
error, operating in opposite sense on opposite 
sides of the piston, yet there is a difference in 
the work done upon the opposite sides because 
of the commencement of the two cycles of 
operation at different times, whence we infer 
differences in the speeds of the piston, in the 
quantities of steam admitted, and in the ex- 
pansion-curves. 

In some cases such a difference is desirable 3 
and with the working diagram suggested in 
Chapter I. another series of results may be 
obtained by using the ordinary valve for dis- 
tribution on one side of the piston, and for 
the other side using a differently-proportioned 
valve with a different index-pointer, as though 
opposite ends of one valve were differently 
proportioned, as in certain large vertical en- 
gines. 

Adding Extra Lap to a Valve. — In some 
cases cut-off may be wanted earlier in the 
stroke, permanently, with expansion of longer 
duration. Under such circumstances more lap 
may be added to the valve. If this be done, 



64 The Slide Valve. 

and the valve with its extra lap be re-set by 
giving the eccentric greater advance in order 
to get the same lead as before (travel of valve 
remaining unaltered), the steam-ports will not 
be opened to so great an extent as formerly, 
and will close earlier, partly because of the 
added lap,* and partly because of the increased 
advance. The increased length of the valve- 
face will, after cut-off, confine the steam longer 
in the cylinder, and it should be obvious (the 
effect of advancing the eccentric having al- 
ready been shown with reference to fig. 32) 
that in addition to the earlier commencement 
and lengthened duration of expansion, the ex- 
haust and compression will begin somewhat 
earlier because of the increased advance, and 
the compression will be of longer duration 
because of the increased length of the valve- 
face. 

General. — It is here seen that in altering 
either the outside or the inside laps with intent 
to affect only the steam admission with the 

* To assure one's self of this, take an extreme case and 
imagine an excessive amount of lap to be added. In such a 
case, with travel unaltered, the ports might not be opened to 
steam at all, the valve simply sliding backwards and forwards 
without uncovering them ; now, taking this lap off piecemeal 
the port would first be opened to an exceedingly small degree 
and instantly shut again, the amuont and duration of opening 
increasing with each successive removal of lap. 



Alterations. 65 

former, or only the exhaust with the latter, 
we must perforce affect other things. For in- 
stance, we alter the outside lap, with the in- 
tention, say, of simply making cut-off earlier 
or later as the case may be — but having altered 
the lengths of the valve-faces, they will alter 
the duration of expansion and of compression, 
and the alteration necessitated in the advance 
of the eccentric in order that the lead may 
remain unaltered has its own effect in addition. 
This interdependence, which is often incon- 
venient, is characteristic of the slide valve 
worked by an eccentric, and has led to the use 
of valve gears and valves which permit, more 
or less, of the independent regulation of some 
or all of the various operations in the distribu- 
tion of steam. Some of these gears are used 
with a slide valve (as in the case of the Joy 
and other well-known valve gears), while 
others operate special valves. 



CHAPTER VIII. 

NOTE ON LINK MOTIONS. 

The Link Motion. — In engines whose duty 
frequently varies, as in the familiar case of 
a locomotive, permanent alteration of lap 
would not meet the requirements of the case, 
for the amount of expansion should be sus- 
ceptible of ready variation at any time, in ac- 
cordance with alterations in the weight of 
train, gradients, and weather. The link mo- 
tion, in addition to being a reversing gear, is 
also a "variable expansion" gear. A precise 
analysis of any given case of link motion is 
a somewhat difficult operation, but, speaking 
broadly, the link motion may be said to pro- 
vide what is virtually an eccentric of different 
throw and advance for each degree of expan- 
sion. Although, actually, the same two eccen- 
trics remain constantly in use, the link motion 
may be so adjusted as to have an effect on 
the valve similar to that which would ensue 



Note on Link Motions. 



6 7 



upon the substitution, one for another, of dif- 
ferent eccentrics as stated. Assuming that the 
lowest range of expansion is being used, the 
valve will have the travel, cut-off, and lead due 
for the most part to the throw of the actual 
eccentric and to its position with relation to 
the crank, almost as in the case of an eccentric 
whose rod is directly connected to the valve- 
spindle ; now, as the link is raised (an opera- 
tion sometimes termed " linking-up ") its cen- 
tre gets nearer to the block on the end of the 
valve-spindle than it was before {i.e., nearer 
to "mid-gear"), and we get in effect: — 

A series of eccentrics of de- 
creasing throw, but of 
increasing angular and 
linear advance. 



With link mo- 
tion having ^ 
"open" rods. 



With link mo- 
tion having 
" cro s s e d" 
rods. 



A series of eccentrics of de- 
creasing throw and of 
increasing angular, but 
decreasing linear ad- 
vance ; throw, therefore, 
decreasing more rapidly 
than in the case of the 
link motion with " open " 
rods. 



Take the case of a partially linked-up mo- 



68 



The Slide Valve. 



tion with open rods ; we shall find that because 
the throw of the eccentric is lessened, the 





^j-r-if*.-^-J\k~j -, M UK>iXmUU with)- 




T*f**r* 



Fig. 34 . 



travel of the valve to the right and left of its 
central position will be diminished, and the 



Note on Link Motions. 



69 



ports will not be opened so wide to steam or 
exhaust ; because of the increased advance of 
the eccentric, steam will be cut off earlier, 
compression and lead will also be earlier, ex- 
pansion and compression lasting longer, and 







*&t 



Fig. 35. 



exhaust and admission being shortened. The 
difference between working in full gear and 
linked up is shown by the diagram in fig. 34. 

Below are diagrams showing the effect of 
putting the link motion in " mid-gear " (fig. 



Note on Link Motions. 



71 



35) and of putting the link motion into " back 
gear " (fig. 36) while the engine is running 



Adm. Expan. Exh. Comp. Ld. 



Old 
Valve. 

Do. do. 

with 
Lead. 

Valve 

with 

Lap and 

Lead. 

Inside 
Lead. 



Inside 
Lap. 

Extra 
Advance. 

Shifted 
Valve, 
left-hand 
Port. 
Right- 
hand do. 

Link 
Motion. 




FIG. 37. 



ahead, as for a locomotive, but it will be 
understood that the effect of the compression 
shown in the latter will be modified by the 



72 The Slide Valve. 

lifting of the valve from its face, with the 
result that some compressed air and steam will 
go up the exhaust. 

The set of diagrams forming fig. 37 is in- 
tended to give a comparative view of the effect 
upon the distribution of steam, of differently 
arranged valves and eccentrics. The opera- 
tions taking place upon only one side of the 
piston are shown, except in that section of the 
diagram which concerns a shifted valve, in 
order to avoid confusion. The upper line of 
each diagram, being read from left to right, 
shows the operations which take place on the 
forward stroke, those of the return stroke be- 
ing read from right to left upon the lower por- 
tion of each diagram. 



CHAPTER IX. 

NOTE ON VERY EARLY CUT-OFF, AND ON REVERS- 
ING GEARS IN GENERAL. 

Very early Cut-off. — Theoretically there is no 
limit to the range of cut-off which may be ob- 
tained from an ordinary slide valve, but a 
moment's consideration will show that if we 
require a valve to cut off steam very early in 
the stroke of the piston so as to cause it to ex- 
pand throughout an increased fraction of that 
stroke, the lap must be considerably increased 
beyond that which is usual, if the valve-face 
is to be enabled to keep the steam confined in 
the cylinder for a sufficient period after cut-off 
is effected. 

As a consequence of increased lap, the travel 
of the valve, made up of lap phis the required 
port-opening, will need to be increased corre- 
spondingly ; the linear advance (the sum of lead 
plus an increased lap) will be increased also, 
and the valve will therefore come back earlier 



74 



The Slide Valve, 



into its central position (where compression be- 
gins) than would the normal valve, so making 
compression begin earlier and last longer than 
before. In an unbalanced valve, this increased 
travel, together with the abnormal compres- 
sion, cannot be tolerated, so that for obtaining 
an early cut-off without too much compres- 
sion, with a sliding valve, gear of the " sepa- 
rate cut-off slide," or Meyer type, is em- 
ployed. This is characterised by a cut-off 



y^^^^ 




FIG. 38.— Slide Valve, with Cut-off Plate on back. 



plate or plates on the back of a main slide 
valve, which may have the usual exhaust 
cavity, and in addition possesses supply steam- 
conduits leading through the valve from the 
back to the " lap-edges '' (fig. 38). Exhaust 
and compression are timed by the edges of the 
central exhaust cavity ; lead is given by the 
" lap-edges," but cut-off is controlled by move- 
ment of the cut-off plates on the back of the 
moving main valve, so that though a " lap- 



Very Early Cut-off. 75 

edge " may not at any time have closed one of 
the steam-ports proper, a cut-off plate worked 
by a separate eccentric can close the appro- 
priate steam-conduit above referred to, and so 
stop the current of steam flowing past the 
lap-edge to the still open port. Moreover, by 
a suitable setting of the eccentrics of the main 
and cut-off valves, it is possible to arrange that, 
at the instant of cut-off, the two valves shall 
be moving in opposite directions, and so shall 
effect a sharp cut-off, whereby wire-drawing 
is reduced. 

With the model referred to in the earlier 
chapters, the action of a Meyer or similar gear 
can be readily investigated if an extra eccen- 
tric-arm be added to the crank-pin disc and 
an extra travel-scale marked upon the base- 
plate. 

In fig. 39 is shown the " Buckeye " valve 
used in America on the Buckeye engine ; a is 
the main valve, which determines the admis- 
sion, release and compression of the steam ; 
and b is the small auxiliary valve which rides 
on the top of the main valve a, and which ef- 
fects the cut-off. This valve is constructed so 
as to have all the advantages of a perfectly 
balanced valve as well as those of the Meyer 
valve. There are two faces betw r een which the 



The Trick Valve. JJ 

main valve slides steam tight, and the small 
valve works on a face inside of the main valve, 
as shown in the figure. Steam is exhausted 
past the outer edges of the main valve. In 
order that steam may enter the cylinder, the 
port in the main valve must be in communica- 
tion with the steam port and at the same time 
it must not be covered by the auxiliary valve b. 
In the figure the valves are shown in such a 
position as to admit steam to the left-hand end 
of the cylinder and to allow it to exhaust from 
the right-hand end, as indicated by the arrows. 

Each valve is moved by its own eccentric, 
quite independently of the other. By changing 
the advance or eccentricity, or both, of the 
auxiliary valve, the cut-off may be changed as 
desired, without in any way affecting the mo- 
tion of the main valve. And as the main valve 
determines the admission, release, and com- 
pression of the steam, it is possible, with this 
valve, to change the cut-off as desired without 
affecting these at all. 

The Trick or Allen Valve. — In connection 
with the subject of early cut-offs the Trick or 
Allen valve may be mentioned. It is made 
with a passage-way through the back of the 
valve, as shown in fig. 40, adapted to deliver 
an extra supply of steam into the port by ren- 



78 



The Slide Valve. 



dering available an additional proportion of 
the area thereof, when the narrow port-open- 
ing given by the lap-edge alone, during 
linked-up working, is found to wire-draw the 
steam. Whilst the lap-edge a, for instance, 
uncovers the edge b of the right-hand port, 
the left-hand end of the Trick passage becomes 
uncovered by moving beyond the edge c of the 
port-face, so that a current of steam flows 
from left to right through that passage to join 



y/y////////y///y////z { oy/^//7Z 




/ ///////////////Z7/7^/Z ; < 

\ 

i 




Fig. 40. 



the stream passing the edge a. The total area 
of the two supply-streams flowing into the 
right-hand port being practically twice that 
which would be available were the Trick pas- 
sage absent. 

The occurrence and duration of the opera- 
tions of steam-distribution are exactly the same 
with this valve as with an ordinary slide-valve 
of corresponding proportions, the distinctive 



Reversal. 79 

feature of the valve having no effect upon them 
so far as their timing is concerned. 

Reversal. — It may be of interest to consider 
the question — " Why do we need reversing 
gears ? " The gears in common use are nearly 
all variable expansion gears, as well as being 
reversing gears, but it is desired to consider 
now only the question of reversing. Are re- 
versing gears really needed? Suppose that 
one could have a transparent cylinder and 
valve-chest, so that movement of the piston 
and valve could be clearly seen. Suppose 
also that the connecting-rod, crank, valve gear, 
and fly-wheel could not be seen. We look into 
the cylinder and see valve and piston recipro- 
cating regularly, and in harmony with one 
another. We turn away, and before we look- 
again, the engine, let us assume, has been re- 
versed without our knowledge ; yet when we 
turn again to the engine there are the piston 
and valve reciprocating just as they were at 
first (although they must have u changed 
step," as we shall presently find), and, look as 
intently as we may, with nothing else visible 
we shall be unable to tell that the engine has 
been reversed. The valve is doing now exactly 
what it did before, performing its operations 
in the same order. Whv have we had to em- 



80 The Slide Valve. 

ploy reversing gear? Solely because — when 
the engine ceases to run ahead for instance, 
stopping at half-stroke the piston and valve 
will be in a certain position, and will have been 
moving in certain directions relatively to one 
another. And to reverse the piston's motion, 
we want the valve 

Firstly — To take up a new position on the 
port-face, so as, for example, to give steam 
where it had previously given exhaust, and 
thus we get reversal of the engine; and 

Secondly — To move away from that new 
position, after the engine has started astern., 
in the same direction as that in which it was 
moving when the engine was running ahead. 
And seeing that the valve gear derives motion 
from the crank-shaft directly or indirectly, we 
cannot give the valve motion in this same di- 
rection from an engine now moving reversely, 
unless we also reverse the gear. And thus we 
may conclude that a reversing gear is needed 
simply for the purpose of regulating the direc- 
tion of the first motion of the valve after 
stopping {i.e., for " changing the step"), for 
after it next reaches the end of its travel its 
motion will be in all respects just as it was 
when the engine ran ahead, — so that, if we 
had a single-cylinder engine and never stopped 



Key to Disc. 81 

it except on the dead centres, we could drive 
its valve by a separate valve-engine ahvays 
running one may, and our main engine would, 
if pushed off by hand, run equally well either 
ahead or astern, whilst served by a valve hav- 
ing lap, lead, and travel as is customary. It 
is hoped that this little speculation may not 
appear unprofitable, for it has a bearing on the 
action of the reversing slides in radial valve 
gears such as those of Hackworth, Walschaert, 
and Joy. 

FIG. 41 SHOWS ENLARGED KEY-VIEW OF CARD- 
BOARD CRANK-DISC, WHICH SHOULD TURN 
WITHIN THE CIRCLE DRAWN IN THE CENTRE 
OF FIG. I. 

The arm marked C is the crank-arm, and 
serves as an index finger in the recording and 
reading of results. 

The mark 1 is the index-mark for the for- 
ward eccentric to operate a valve without lap.* 

The mark 2 is the index for a forward eccen- 
tric set for lead, using a valve without lap.* 

The mark 3 is for a forward eccentric for 
a valve with lap and lead run in " full gear."f 

* Used with No. i travel-scale, fig. i. 
t " " No. 2 " 



82 



The Slide Valve. 



The mark 4 is for a forward eccentric for 
a valve with lap and lead run with a link mo- 
tion partially "linked up."X 



12 




98 



Fig. . 



The mark 5 is for a valve with lap and lead 
run with the link motion in mid-gear.* 

The mark 6 is for a backward eccentric for 



X Used with No. 2 travel-scale, fig. 1. 
* " . " No. 1 



Key to Disc. 83 

a valve with lap and lead, run with a link mo- 
tion partially linked-up4 

The mark 7 is a backward eccentric for 
a valve with lap and lead run in " full gear."f 

The mark 8 is for a backward eccentric set 
for lead, using a valve without any lap.* 

The mark 9 is for a backward eccentric for 
a valve without lap.* 

The radial lines between the centre of the 
disc and the small circles represent the actual 
throw assumed for the various eccentrics and 
their angular position in relation to the crank ; 
where the actual throw is not sufficient to en- 
able the arms to reach the edge of the disc, 
the throw-lines are produced to the edge. 

X Used with No. 2 travel-scale, fig. 1. 

t " " No. 2 

* ** " No. 1 " " ** 



THE 



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SMALL ACCUMULATORS 

How Made and Used 

A Practical Handbook for Students and Young 
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EDITED BY PERCIVAL MARSHALL, A.I.M.E. 



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II. — How to make a 4-Volt Pocket Accumulator. 
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THE MAGNETO -TELEPHONE 

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BY NORMAN HUGHES 



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Induction Coils 

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BATTERIES, TESLA COILS AND 
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THE VOLTAIC ACCUMULATOR 

AN ELEMENTARY TREATISE 

BY EMILE REYNIER 

TRANSLATED FROM THE FRENCH 
BY J. A. BERLY, C.E., A.I.C.E. 



CONTENTS 

Part I. — Principles. Definitions. Voltameters. Classi- 
fication of Accumulators. 

Part II. — Voltaic Accumulators. Plante Accumu- 
lators. Accumulators of the Plante type. Autogeneous 
formation. Heterogeneous formation. Miscellaneous 
Accumulators. 

Part III. — Technology. Technical generalities con- 
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Accumulators. 

Part IV. — Applications of Accumulators. General re- 
marks on the charge of Accumulators. First applications of 
secondary couples by Mr. G. Plante. Applications to 
scientific researches, to telegraphy, telephony, clockwork, 
etc. Application to electric lighting, and to the current 
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of secondary currents. Translation or displacement of 
energy by means of Accumulators. Land carriage of en- 
ergy. Water carriage of energy. Caption of natural forces 
by means of Accumulators. 

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